大港滩海埕北断阶区沙河街组三段、二段近岸水下扇沉积微相特征

大港滩海埕北断阶区沙河街组沙三段、沙二段砂体是由辫状水道控制的近岸水下扇。主要发育基质支撑砂砾岩相、颗粒支撑砂砾岩相、正递变层理砾岩相、含砾砂岩相、逆递变层理砾岩相、含砾砂岩相、泥质支撑砂岩相和粉砂岩相，粒度粗是其主要特征；水平层理、块状层理、平行层理、交错层理、波状层理、包卷层理和变形层理十分发育；砂球、重荷模、砂岩脉和冲刷面等沉积构造也十分发育：电阻率曲线呈正韵律，曲线由高幅至中幅呈钟形及箱形，与不同岩性和不同沉积相具良好对应关系。通过对区内各井的岩性组合观察，结合各砂层组的沉积特征及其相应的测井相标志将研究区近岸水下扇划分为2个亚相和5个微相：扇中(包括辫状水道、辫状水道间和扇中前缘)和扇端亚相(扇端泥、扇端席状砂)。其中扇中辫状水道为主要储集层。

Characteristics of sedimentary microfacies of the third and second members of Shahejie Formation in nearshore subaqueous fan in Chengbei step-fault zone, Dagang offshore area

Sand body in the third and second member of the Shahejie Formation in Chengbei step-faut zone of Dagang off-shore area is the nearshore subaqueous fan controlled by braided stream. The petrographic facies including matrix-supported gravelly sandstone,grain-supported gravelly sandstone,normal grading bedding pudding stone,gravelly sandstone,inverse grading pudding stone,gravelly sandstone,argillaceous-suppoted sandstone and siltstone are mainly characterized by coarse graind sedimention;depositional bedding such as level bedding,massive bedding,parallel bedding,cross-bedding,wave bedding,convolut bedding and deformed bedd- ing are very developed;sand ball,load cast,sandstone vein and scoured base are also developed very well;electrical resistivity log shows positive rhythm with bell-tepy and box-like shapes changing respectively from high amplitude to medium amplitude which are closely related to different lithology and different depositional facies. On the basis of lithology association,sediment character and well logging,the nearshore subaqueaus fan in this area can be subdivided into two subfacies and five micro-facies:fan-middling including braided stream,spacebetween braided stream and fan-middle leading edge;fan-end include fan-end mud and fan-end sheet stone; braided stream is main reservoir.